1. Field of the Invention
This invention relates to a furnace, and more particularly a furnace for firing a glass-lined product.
2. Description of the Prior Art
There have been provided various furnaces which are categorized broadly into a burner type and an electrical type in heating system, and a batch type and a continuous type in operation system. Among these types, a suitable type is selected in accordance with a kind of an article to be fired.
A glass-lined product is manufactured through a process of spraying a ground coat or a cover coat on a metal surface of the article, drying and firing such coated article. More particularly, the article which has been coated and dried is generally fired by a quick heating within a furnace chamber, and subsequently cooled after it is withdrawn from the furnace chamber.
If contamination is caused on a glass surface of the article during repetition of the spraying, drying and firing process, corrosion resistance of a finished glass-lined product may be deteriorated.
Accordingly, atmosphere within the furnace chamber must be kept in clean state to properly fire the glass-lined product. However, the furnace of the burner type involves a problem that combustion gas of a burner may cause some damages against the glass-lined surface of the product, and further cause foreign matters to be spread over inside of the furnace chamber and stuck on the article.
As far as is known it is essential that the agitation is not performed within the furnace chamber to avoid the foreign matters from spreading over the inside of the furnace chamber, which may cause deterioration of the glass-lined product, and that a uniform temperature distribution is provided throughout the entire region of the furnace chamber, when the glass-lined product is to be fired within the furnace chamber of the burner type.
In view of the foregoing considerations and problems, it was a common practice that the glass-lined product is fired by the electric furnace.
As an additional limitation in selecting the furnace of the type suitable for firing the glass-lined product, the glass-lined product does not commonly have a uniform construction, but a customized construction which is not suitable for mass-production, where a lot of the articles with uniform construction are successively fired. Accordingly, the glass-lined product is generally fired by the batch type furnace.
However, the electric furnace involves a problem that it rises an energy cost, and therefore is not economical in operation.
To keep the atmosphere within the furnace chamber of the burner type in clean state, a construction with a radiant tube is employed in some cases, where a centrifuged-casting pipe is commonly used as a material for the radiant tube. However, the centrifuged-casting pipe is usually formed by a thicker wall which may reduce a quick heat transfer and increase the weight of the radiant tube, which renders the radiant tube of the centrifuged-casting pipe unsuitable for firing of the glass-lined article, because a high precision temperature control is required to quickly heat the glass-lined article to a temperature at which the glass-lined article is fired, and perform other heating operations.
As an inherent characteristic of the batch type furnace in operation, the inside of the furnace chamber is cooled whenever the article is placed into and out of the furnace chamber. Such quick heating and cooling required in the firing of the glass-lined article causes a relatively large temperature difference for every batch, and such temperature difference frequently and repeatedly occurs. As a result, it was likely that the centrifuged-casting pipe splits due to temperature stress in a relatively short period of time.
To solve the problems as described above, it is an object of the present invention to provide a furnace of the burner type being capable of exhibiting a high heat efficiency, proceeding the heating operation under clean atmosphere, applying heat uniformly within the entire region of the furnace chamber, as well as being operated without problems under the condition of the quick heating and cooling operation.